1. Field
The present invention pertains generally to apparatus for monitoring radiation within the core of a nuclear reactor and, more particularly, to such apparatus that is self-powered.
2. Related Art
In many state of the art nuclear reactor systems, in-core sensors are employed for measuring the radioactivity within the core at a number of axial elevations and radial locations. These sensors are used to measure the axial and radial distribution of the power inside the reactor core. This power distribution measurement information is used to determine whether the reactor is operating within nuclear power distribution limits. The typical in-core sensor used to perform this function in modem nuclear reactors is a self-powered detector that produces an electric current that is proportional to the amount of fission occurring around it. This type of sensor does not require an outside source of electrical power to produce the current and is commonly referred to as a self-powered detector and is more fully described in U.S. Pat. No. 5,745,538, issued Apr. 28, 1998, and assigned to the Assignee of this invention. FIG. 1 provides a diagram of the mechanisms that produce the current I(t) in a self-powered detector element 10. A neutron sensitive material, such as vanadium, is employed for the emitter element 12 and emits electrons in response to neutron irradiation. Typically, the self-powered detectors are grouped within instrumentation thimble assemblies within the fuel assemblies. A representative in-core instrumentation thimble assembly is shown in FIG. 2. The signal level generated by the essentially non-depleting neutron sensitive emitter element 12 shown in FIG. 1, is low, however, a single full core length neutron sensitive emitter element provides an adequate signal that can be processed for determining core power at the sensor's location. The proportions of the signal from the full length emitter attributable to various axial regions of the core are determined from apportioning the signal generated by different lengths of primarily neutron sensitive elements 14 which define the axial regions of the core and are shown in FIG. 2. The apportioning signals are ratioed which eliminates much of the effects of the delayed gamma radiation due to fission products. The in-core instrumentation thimble assemblies also include a thermocouple 18 for measuring the temperature of the coolant exiting the fuel assemblies. The electrical signal output from the self-powered detector elements and the thermocouple in each in-core instrumentation thimble assembly in the reactor core are collected at the electrical connector 20 and sent to a location well away from the reactor for final processing and use in producing the measured core power distribution.
FIG. 3 shows an example of a core monitoring system presently offered for sale by Westinghouse Electric Company LLC, Cranberry Township Pennsylvania, with the product name WINCISE™ that employs fixed in-core instrumentation thimble assemblies 16 within the instrument thimbles of fuel assemblies within the core to measure the core's power distribution. Cabling 22 extends from the instrument thimble assembly 16 through the containment seal table 24 to a signal processing cabinet 26 where the outputs are conditioned, digitized and multiplexed and transmitted through the containment walls 28 to a computer work station 30 where they can be further processed and displayed. The thermocouple signals from the in-core instrumentation thimble assemblies are also sent to a reference junction unit 32 which transmits the signals to an inadequate core cooling monitor 34 which communicates with the plant computer 36 which is also connected to the work station 30. Because of the hostile environment, the signal processing cabinet 26 has to be located a significant distance away from the core and the signal has to be sent from the detector 16 to the signal processing cabinet 26 through specially constructed cables that are extremely expensive and the long runs reduce the signal to noise ratio. Unfortunately, these long runs of cable have proved necessary because the electronics for signal processing has to be shielded from the highly radioactive environments surrounding the core region.
In previous nuclear plant designs, the in-core detectors enter the reactor vessel from the lower hemispherical end and enter the fuel assemblies' instrumentation thimble from the bottom fuel assembly nozzle. In at least some of the current generation of nuclear plant designs, such as the AP 1000® nuclear plant, the in-core monitoring access is located at the top of the reactor vessel, which means that during refueling all the in-core instrument thimble assemblies will need to be removed from the core before accessing the fuel. In either arrangement, the long runs of signal cable are necessary to isolate the electronics from the harmful effects of radiation emanating from the core.
Self-powered detectors are generally coaxial in design with a center emitter wire, an annular alumina insulator and an outer metallic sheath. By some physical process the central wire emits electrons, some of which form the detector current. Some electrons slow down in the insulator leading to a space charge therein. The emitter and sheath are more or less at ground potential. The radius in the insulator where the minimum (most negative) potential occurs determines if charges that come to rest within the insulator are counted or not. For example, a Compton electron enters the insulator from the sheath with 300 keV of energy and comes to rest (due to collisions) in the insulator just inside the (probably less in magnitude than −1 volt) minimum potential radius. This particular electron is then directed to the emitter by the potential inside the insulator. As such, it creates a charge flow that subtracts from the total detector current.
Existing self-powered detectors have a reduced sensitivity due to the electrical potential trough that builds up in the insulating annulus. This is caused by a portion of the Compton electrons and beta particles coming to rest in the insulator due to kinetic interactions. The minimum potential formed by these particles is small, perhaps not even minus one volt, but is enough to direct the at rest electrons on the inside of the insulator back to the emitter. These charges are then not counted as they have finally not escaped the emitter. Although the minimum potential is small, it typically occurs at a depth in the insulator that takes 100's of keV of kinetic energy to reach. This then precludes low energy electrons or betas from contributing to the detector current. Similarly, the predominantly low energy photoelectric electrons are not able to penetrate the insulator to a depth on the outer side of the minimum potential. Consequently, they too are not counted.
It is an object of this invention to improve the sensitivity of self-powered neutron detectors.
Furthermore, it is an object of this invention to increase the sensitivity of the self-powered detectors without substantially altering the configuration of existing systems.